Disc brakes are commonly used on both commercial (heavy) and private (light) vehicles. Some are hydraulically operated, while others are air operated. Some disc brakes are operated by pistons acting on one side of the brake. Due to vehicle space limitations it is common to mount such pistons on the inboard side of the wheel in question, as there is generally more space inboard than outboard of a vehicle's brake disc.
Certain disc brake pads wear in a tapered manner. Thus a leading edge of an outboard brake pad and an inboard brake pad will wear more than the trailing edge of those brake pads.
It is known to counter the problem of tangential taper wear by offsetting the inboard pad in the direction of its leading edge and offsetting the outboard pad in the direction of the trailing edge. Such offset pads can be seen in U.S. Pat. No. 3,422,935 (Van House). Moving the pads in this way counters the rotational forces and balances the pads against the brake disc more evenly. A similar arrangement can be seen in U.S. Pat. No. 4,533,025 (Carré). U.S. Pat. No. 3,422,935 does not differentiate between wear on the inboard and outboard pads, referring only to tapered lining wear of both pads. U.S. Pat. No. 3,422,935 maintains that offsetting both pads in this way will lead to equal and opposite force couples around the point where the braking force is applied. U.S. Pat. No. 4,533,025 also refers to the correction of tapering wear of each brake pad.
Offsetting of the inboard pad in the leading direction moves the center of pressure of that pad in the leading direction and hence the center of pressure of the reaction force of the disc in the leading direction. Offsetting the outboard pad in the trailing direction offsets the center of pressure of the outboard brake pad in the trailing direction, hence offsetting the reaction force of the disc against the outboard pad in the trailing direction. The pads therefore create a couple on the disc. The offset center of pressure of the inboard and outboard brake pads also creates unequal forces in the caliper bridge.
U.S. Pat. No. 5,022,500 shows an arrangement wherein the mounting side brake pad has no offset in relation to the piston but the non-mounting side brake pad is offset towards the trailing edge.
JP932870 FIG. 1 shows a mounting side brake pad offset towards the leading edge with respect to the piston and a non-mounting side brake pad offset towards the leading edge with respect to the piston. In FIG. 3 of JP932870 the mounting side brake pad is offset towards a leading edge whereas the non-mounting side brake pad is offset towards a trailing edge with respect to the piston.
FIG. 1 of U.S. Pat. No. 5,386,890 shows mounting side brake pad offset towards the trailing edge with respect to the single piston.
All of the above documents relate to hydraulic brakes on light vehicles such as cars. An advantage of such brakes is their size and weight in comparison to those used on commercial vehicles such as lorries and trucks. Manipulating these smaller, lighter brakes, for example when changing brake pads, is much easier than performing the same operation with their commercial counterparts. The calipers of light vehicles can be partially unscrewed and manually raised away from the disc in order to allow the replacement of brake pads. It does not matter, therefore, if the bridge arm of the caliper or carrier normally impedes access to the brake pads. In contrast, the brake pads of commercial vehicles should be accessible without the need for removal or manipulation of the much heavier carriers used (since these carriers are so heavy that they cannot safely be manually handled by a single person), and hence they incorporate spaced caliper bridge arms between which the pads can be removed and replaced without having to disturb the caliper fixings.
Because commercial vehicle brakes have to withstand far higher forces, they tend to be heavier. Typically a commercial vehicle brake might include one or more, or all, of the following features:                a) a fixed carrier which reacts the tangential (braking) forces from both the mounting side pad and the non-mounting side pad,        b) a sliding caliper which is designed to apply forces to the mounting side brake pad and non-mounting side brake pad so that the brake disk is clamped and therefore braked,        c) the floating caliper acting so as to not transfer the tangential braking loads to the vehicle (this function being carried out by the fixed carrier),        d) mounting side and non-mounting side brake pads which are removable through spaced arms of the carrier,        e) mounting side and non-mounting side brake pads which are removable through spaced arms of the caliper.        
Tapering pad wear can also occur on heavy vehicle brakes, for example commercial vehicle brakes such as those used on lorries and trucks.
US2007/0256900 shows a mounting side brake pad aligned with the piston and a non-mounting side brake pad offset towards a trailing edge. U.S. Pat. No. 7,461,725 is similar in this respect.
What is required is a way to reduce tangential tapering wear of the mounting side pad (e.g. the inboard side pad when the brake operating piston is mounted on the inboard side of the brake disc) of a commercial disc brake that minimizes the creation of imbalanced forces in the brake caliper or in the brake disc, in particular in the spaced bridge arms. This is particularly important since the clamp forces involved when using commercial vehicle disc brakes are far higher than the clamp forces involved on disc brakes of lighter vehicles such as cars and motorcycles.